This invention relates to the casting of metals to make thin-wall articles, and, more particularly, to the casting of such articles using metals which are subject to hot tearing.
Many articles are made by casting molten metal into a mold that closely defines the final shape of the article, and solidifying the molten metal in the mold. The solidified metal may need some final processing such as consolidating, crack repairing, cleaning, and/or final machining, but its gross as-cast shape is that of the desired final article.
This casting process is sometimes used because, for some metals, there is no practical alternative. These same metals have such low ductilities, even at elevated temperatures, that they cannot be fabricated by conventional metal working procedures. One casting application is the fabrication of parts used in the high-temperature portions of gas turbine (jet) engines. An example of such an article is the mixer used in some gas turbine engines to mix ambient air with exhaust gas.
Some of the metal alloys that would be desirably used in these castings are subject to hot tearing when cast by conventional procedures. Two examples are the nickel-base superalloys Rene' 108 and Mar-M247. These metals are qualitatively different from other nickel-base superalloys, in that they exhibit low grain boundary ductilities in the range from below the solidus temperature (about 2434.degree. F. in these alloys) down to about 2100.degree. F., as well as low polycrystalline grain boundary strengths in this same temperature range. During the cooling through this temperature range that follows casting of the molten metal and initial solidification, differential strains caused by the difference in thermal expansion coefficient between the metal and the surrounding casting shell mold create large differential stresses within the metal and cause grain boundary (intergranular) cracks to open. This cracking phenomenon is termed "hot tearing". The hot-tearing cracks remain in the final article, in many cases causing it to be unacceptable and in other cases requiring expensive repair and reworking before the article is acceptable for service.
The problem is particularly troublesome when metals subject to hot tearing are used to cast large, thin-walled structural articles by conventional processes. Localized differential strains across the wall thickness may be quite large, and grain boundary hot tearing is often observed. One approach to alleviating the hot tearing and defects in the casting is to modify the composition of the alloy to improve grain boundary ductility, but then. the desirable final mechanical properties of the material would be lost as well.
There is a need for an operable approach to the casting of large, thin-walled and hollow articles made of metals that are subject to hot tearing. The present invention fulfills this need, and further provides related advantages.